The present invention relates to magnetic-field apparatus for a particle accelerator, the particle track of which has at least curved sections, with several magnetic field-generating windings, wherein at least one supplemental winding for focusing the electrically charged particles is provided. Such apparatus is known, for instance, from the publication "Nuclear Instruments and Methods", vol. 203, 1982, pages 1 to 5.
With known, smaller electron accelerators of circular shape which are also called "microtrons", particle energies up to about 100 MeV can be achieved. These systems can be realized particularly also as so-called "race track" microtrons. The particle tracks of this type of accelerator are composed of two semi-circles each having one 180.degree. deflection magnet and further having two straight track sections (see "Nucl. Instr. and Meth.", vol 177, 1980, pages 411 to 416, or vol. 204, 1982, pages 1 to 20).
If the desired final energy of the electrons is to be increased from 100 MeV to, for instance, 700 MeV, increasing the magnetic field is available, with no change in the dimensions. Such magnetic fields can be generated particularly with superconducting magnets.
If, however, low-energy electrons are injected into a microtron, which in addition, can further comprise superconducting magnet windings with a very low magnetic field, a number of possible field error sources must be noted in order to keep the electron losses during the acceleration phase low. For example, at the beginning of this phase, the field level for electrons injected at a low energy of, for instance, 100 keV, is only about 2.2 mT with a radius of curvature of the accelerator of, for instance, 0.5 m. However, with such low magnetic field intensities or with high field-change rates, the danger then exists that, due to field-distorting interference sources, the field error limits which are to be kept, may be exceeded. In order to be able to guide an electron beam through weak focusing, a field accuracy .DELTA.B/B.sub.0 of about 10.sup.-3 would be required; this means that the field at the beginning of the acceleration phase must be adjustable to an accuracy of about 0.002 mT. Then, however, the cause of undesired field distortion can be external fields such as the Earth's field with 0.06 mT, or the field of magnetizable, i.e., para-, ferrior ferro magnetic parts of a magnet system. Also, eddy currents in metallic parts of the magnet itself or in its conductors can lead to corresponding disturbances. In addition, shielding currents in the conductors of a superconducting winding or so-called frozen magnetic fluxes in these conductors can constitute such error sources.
It has been attempted to eliminate difficulties resulting from such interference field sources, for instance, by shielding or compensation of the interfering field. Thus, it is attempted in known electron accelerators with normal-conducting copper coils to obtain a shielding effect by means of a flux return of iron. In addition, laminating the iron yokes of the field-generating magnets is known for suppressing the formation of eddy currents. Possibly, a field reversal can also be performed in order to traverse the hysteresis curve of the iron of the magnetic apparatus reproducibly.
A further difficulty arises if relatively large particle streams are to be produced and the particles are to be injected into the accelerator track with relatively low energy. This is because the repulsion forces acting between the individual particles (space charge forces) are relatively dominant; i.e., the particle stream attempts to diverge to a corresponding degree. One is therefore compelled to provide additional measures for focusing the particle beam. In the electron accelerator known from the literature reference mentioned above, "Nucl. Instr. and Meth.", the 180.degree.-deflection magnets with a main winding generating a dipole field also comprise a supplemental winding focusing the particles onto the particle track. In addition, a focusing solenoid system is provided in the region of the straight track sections. In the known magnetic apparatus, however, the deflection magnets enclose the respective curved section of the particle track so that the synchrotron radiation occurring there cannot be utilized.
Because of the interference effects on low-energy particle beams occurring especially if superconducting deflection magnets are used, the particles are generally injected only at higher field level, i.e., with higher energy, since then the mentioned interference effects are only of smaller or secondary importance. Such a mode of operation of the accelerators necessitates appropriate pre-accelerators and is therefore accordingly expensive.